In recent years, the advances in the high density recording of magnetic recording media are remarkable. For example, ferromagnetic metal thin film type magnetic recording media having extremely high recording densities in comparison with conventional magnetic recording media have been developed and put to practical uses. In each of the ferromagnetic metal thin film type magnetic recording media, a ferromagnetic metal thin film has been formed on a non-magnetic recording support (base film) by a physical deposition method such as a vacuum deposition method or a sputtering method or by a plating method. As concrete ferromagnetic thin film type magnetic recording media, a Co-deposited tape has been proposed in JP-A 54-147010 (hereunder, JP-A means “Japanese Unexamined Patent Publication”), and a vertical magnetic recording medium comprising a Co—Cr alloy has been proposed in JP-A 52-134706.
Conventional magnetic recording media are magnetic recording media (coating type magnetic recording media) each obtained by mixing magnetic powder with an organic polymer binder and then coating the mixture on a non-magnetic recording support. The conventional coating type magnetic recording media have large magnetic layer thicknesses of not less than about 2 μm, and low recording densities, and are also long in recording wavelengths. On the other hand, the ferromagnetic metal thin film type magnetic recording media have very small magnetic layer (the metal thin film layer of the magnetic recording medium) thicknesses of not more than 0.2 μm, and high recording densities, and are also short in recording wavelengths.
By the way, the surface property of the magnetic layer of a magnetic recording medium is liable to be affected by the surface state of a base film, when the thickness of the magnetic layer is small. Hence, a ferromagnetic metal thin film type magnetic recording medium having a thin magnetic layer in comparison with a conventional coating type magnetic recording medium is liable to be affected by the surface state of a base film. Concretely, even a base film not having caused a problem in a conventional coating type magnetic recording medium develops the fine unevenness of the surface of a base film in a ferromagnetic metal thin film type magnetic recording medium as such on the surface of the magnetic layer, thereby often producing noises in recording•regenerating signals. Therefore, it is desirable that the surface of a base film used for a ferromagnetic metal thin film type magnetic recording medium is smooth as much as possible.
On the other hand, it is preferable that a base film has excellent slipperiness from the view points of handling properties such as a transporting property, a winding-up property and a winding-off property, in production processes (a film-producing process and a processing process). When the surface of the base film is too smooth, the mutual slipperiness of the film-film is deteriorated, or the surface of the base film is liable to be damaged. When the mutual slipperiness of the film-film is deteriorated or when the surface of the base film is liable to be damaged, the yield of the obtained product is lowered, and the enhancement of the production cost is further caused. Therefore, it is desirable that the surface of the base film is rough as much as possible from the viewpoint of the production cost.
In addition, in a process for producing a metal thin film type magnetic recording medium, a base film is usually subjected to an ion-bombarding treatment to enhance the adhesivity of the metal thin film to the base film. The ion-bombarding treatment is a treatment for activating the surface of the base film with ions, before the thin metal film is formed. When a metal thin film is formed on the surface of a base film, considerably high temperature heat is added to the base film. Hence, the back side of the base film is cooled so that the melting of the base film or the deterioration in the mechanical characteristics and the like of the base film are not caused, when the metal thin film is formed on the surface of the base film. As a method for cooling the back side of the base film is usually adopted a method for winding up the base film on a drum-like cooling member. At the time, both the ends of the base film are masked to prevent the formation of a metal thin film on the surface of the cooling member.
Thereby, portions on each of which a metal thin film has not been formed due to the masking exist in both the end portions of the laminate prepared by forming the metal thin film on the surface of the base film. The portions on which said metal thin films have not been formed continuously exist in the longitudinal direction of the base film, and the surfaces of the portions have been activated by the above-described ion bombarding treatment. When the above-described laminate is wound up in a roll-like shape, the portions on which said metal thin films have not been formed are brought into contact with the opposite side with large forces, and are liable to cause a blocking phenomenon, further because the surfaces of the portions have been activated. When a metal thin film type magnetic recording medium is produced, a metal thin film is vapor-deposited, and, if necessary, processing processes for disposing a back coat layer and a top coat layer are then applied. When the above-described blocking phenomenon is caused, the breakage of a base film and the wrinkling of the base film are liable to be caused in the processing processes, thereby largely lowering the yield of the metal thin film type magnetic medium. Therefore, it is preferable that the surface of the base film is rough for the prevention of the blocking of the film in the process for producing the metal thin film type magnetic recording medium.
Thus, the surface of the base film is required to be smooth from the viewpoint of the electromagnetic transducing characteristics, while being required to be rough from the viewpoints of the handling property improvement, production cost reduction and blocking phenomenon prevention.
In order to satisfy such the conflicting requirements as described above has been proposed a laminated film (for example JP-B 1-26338 (hereunder, JP-B means “Japanese Examined Patent Publication”)) comprising two layers, wherein one of layers is a layer (flat surface layer) whose surface is flat, while the other is a layer (rough surface layer),whose surface is rough. However, this laminated film has problems such as a problem that high projections on the surface of the rough surface layer are transferred to the surface of the flat surface layer and a problem that large particles added to the rough surface layer are pushed up from the inner portions of the laminated film to the flat surface layer to roughen the surface of the flat surface layer.
Additionally, as a method for smoothening the surface of the flat surface layer, it has been proposed to prevent the deterioration in the smoothness of the surface with the residues (fine particles) of a catalyst. Concretely has been proposed a laminated film (for example, JP-A 12-15695) using as the raw material of a flat surface layer a polyester which uses a germanium compound as a polymerization catalyst and contains the germanium and phosphorus in specific amounts. However, this laminated film has a problem that foreign matters such as projections dropped from the rough surface layer or oligomers bled out from the rough surface layer are adhered to the flat surface layer to deteriorate the smoothness of the flat surface layer, in a process for producing a base film or in a process for processing the above-described metal thin film.